fix(chapters): address review findings + trickplay polish

Copilot + CodeRabbit review findings:
- React.memo ChapterTicks and ChapterList (project guideline: hot-path
  components must use React.memo to cut redraw work during control
  updates).
- chapterNameAt now sorts the chapter array once instead of twice per
  call. The previous version went through currentChapterIndex
  (chapterStartsMs + sort) then sortedChapters (sort again). Runs on
  every playback tick, so the duplicate work added up.
- Import getUserLibraryApi from the public barrel
  (@jellyfin/sdk/lib/utils/api) instead of the deep internal path
  (@jellyfin/sdk/lib/utils/api/user-library-api) to match the rest of
  the codebase and avoid coupling to SDK file layout.

TrickplayBubble polish:
- Sit just above the slider (bottom: 0) so the bubble no longer overlaps
  the progress bar.
- Move the chapter-name + timestamp overlay to the bottom-left of the
  preview frame, smaller font, in front of the surrounding overlays
  (zIndex + elevation).

BottomControls cleanup:
- Drop dev-only "pick one to test" comment in favour of a one-line note
  on TICK_HEIGHT.
- Inline scrubMs into its useMemo callback so the scrub-chapter-name
  lookup only recomputes while a slide is active.

components/chapters/ChapterList.tsx

@@ -6,7 +6,7 @@
 
 import { Ionicons } from "@expo/vector-icons";
 import type { ChapterInfo } from "@jellyfin/sdk/lib/generated-client/models";
-import { useEffect, useMemo, useRef } from "react";
+import { memo, useEffect, useMemo, useRef } from "react";
 import { useTranslation } from "react-i18next";
 import { FlatList, Modal, Pressable, StyleSheet, View } from "react-native";
 import { Text } from "@/components/common/Text";
@@ -30,7 +30,7 @@ interface ChapterListProps {
 
 const ROW_HEIGHT = 48;
 
-export function ChapterList({
+function ChapterListComponent({
   visible,
   chapters,
   currentPositionMs,
@@ -151,6 +151,8 @@ export function ChapterList({
   );
 }
 
+export const ChapterList = memo(ChapterListComponent);
+
 const styles = StyleSheet.create({
   backdrop: {
     flex: 1,